1. Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China
2. Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15213, USA
3. Department of Earth Sciences, Nanjing University, Nanjing 200017, China

Correspondence to: Zhe-Xi Luo^2,3 Correspondence and requests for materials should be addressed to Z.-X.L. (Email: luoz@carnegiemnh.org).

Abstract

A new spalacotheriid mammal preserved with a complete postcranium and a partial skull has been discovered from the Yixian Formation^{1, 2, 3} of Liaoning, China. Spalacotheroid symmetrodonts^{4, 5, 6, 7, 8, 9, 10, 11} are relatives to modern therians (combined group of marsupials and placentals) and are characterized by many skeletal apomorphies of therians. But unlike the closely related spalacotheroids and living therians, this new mammal revealed some surprisingly convergent features to monotremes in the lumbar vertebrae, pelvis and hindlimb^{12, 13}. These peculiar features may have developed as functional convergence to locomotory features of monotremes, or the presence of lumbar ribs in this newly discovered mammal and their absence in its close relatives might be due to evolutionary developmental homoplasy. Analysis including this new taxon suggests that spalacotheroids evolved earlier in Eurasia and then dispersed to North America, in concordance with prevailing geodispersal patterns of several common mammalian groups during the Early Cretaceous period.

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